The present invention relates to a radiation detector for use in an X-ray computerized tomography system hereinbelow, referred to as "X-ray CT"), and more particularly to a radiation detector which permits reduction in variations of signal output characteristics from individual cells in a multi-cell ionization chamber radiation detector.
The type of X-ray CT most popularly used at present is what is called a rotate-rotate type. According to the X-ray CT of this type, a tomogram is taken as follows. An X-ray source radiating a fan beam X-ray with an opening degree of nearly 40 degrees is disposed opposite to a multi-cell X-ray detector (hereinbelow, abbreviated simply as "detector") with the distance of about 1 m therebetween. The X-ray source and the detector are revolved about an object placed therebetween while keeping the positional relationship relative to each other. X-rays are irradiated to the object in many directions to measure X-ray intensity distribution, and the measured signals are then processed using a computer to obtain a tomogram.
The defect of this type apparatus is in that a ring-like artifact is apt to occur in the tomogram. The ring-like artifact will be caused in case cells of the detector have their sensitivities different from one another to a certain or more degree. From this reason, it is required to reduce variations in sensitivities among the cells of the detector.
In the X-ray CT using such a tomographic technique, a xenon ionization chamber detector is mainly employed as a detector, one example of which is disclosed in the U.S. Pat. No. 4,161,655.
Pulsed irradiation and continuous irradiation are usually employed in X-ray CT as a method of irradiating X-rays, and pulsed irradiation is employed in the rotate-rotate type apparatus as mentioned above. In the pulsed irradiation, there are repeated, for example, X-ray irradiation of 5 msec and pause of 10 msec.
As a result of measuring output currents from a xenon ionization chamber detector in case of using the pulsed irradiation, it was found that the output current did not become zero after stopping of X-ray irradiation and there occurred a residual current. It was also determined that the residual current had different amounts for each cell depending on the structure of the detector and this caused the aforesaid ring-like artifact.
More specifically, the output currents from individual cells of the detector are integrated by an integrator and then transmitted to an image processing apparatus in the form of signal voltage. It is very difficult to closely coincide the integrating period of the integrator with the period of X-ray irradiation, so that the difference in residual currents will appear as a difference in sensitivities of the cells.